QE 
539.2 


UC-NRLF 


B   M   331   M33 


tM»v.  Ubrory,  Univ.  Calif.,  Santo  Cruz 


[PROM  THE  AMERICAN  JOURNAL  OP  SCIENCE,  VOL.  XXXV,  JUNE,  1888.] 


AKT.  XXXVL-^-Zfofe  on  Earthquake- Intensity  in  San  Fran- 
cisco^ 'by  EDWAKD  S.  HOLDEN,  LL.D.,  Director  of  the 
Lickf  Observatory. 

TOWAKD  the  end  of  1887,  the  Eegents  of  the  University  of 
California  published  a  pamphlet  prepared  by  me  bearing  the 
title  "  List  of  Kecorded  Earthquakes  in  California,  etc. ;"  1 887 ; 
8vo,  pp.  78.  This  work  contained  all  the  information  regard- 
ing California  earthquakes  which  I  have  been  able  to  collect. 
The  information  is  presented  in  a  popular  rather  than  a  scien- 
tific form,  though  the  Introduction  contains  statistics,  more  or 
less  valuable,  relating  to  the  distribution  of  the  shocks  by 
years,  months  and  seasons. 

It  is  the  object  of  the  present  note  to  obtain  an  estimate  of 
the  absolute  value  of  the  earthquake-intensity  developed  at 
San  Francisco  during  our  historic  period.  I  am  obliged  to 
confine  myself  to  San  Francisco,  whose  records  are  very  com- 
plete, owing  to  the  conscientious  care  of  Mr.  Thomas  Tennant. 

With  this  end  in  view  I  have  gone  over  the  printed  pamph- 
let and  wherever  the  data  were  sufficiently  exact,  I  have  as- 
signed the  intensity  of  each  separate  shock  on  the  arbitrary 
scale  of  Rossi  and  Forel,  omitting  every  doubtful  case.  The 
later  papers  of  Professor  Rockwood  already  contained  this 
datum.  Omitting  all  doubtful  cases,  I  found  948  shocks  at 
214  different  stations  in  California  which  had  been  so  well 

AM.  JOUR.  Sci. — THIRD  SERIES,  VOL.  XXXV,  No.  210. — JUNE,  1888. 
26 


428      If  olden — Earthquake-Intensity  in  San  Francisco. 

reported  as  to  allow  an  intensity  on  the  scale,  to  be  assigned 
with  certainty.  In  San  Francisco,  41Y  shocks  in  all  have  been 
recorded.  Of  these,  200  were  accurately  described. 

The  Rossi-Forel  Scale. 

I.  Microseismic  shock — recorded  by  a  single  seismograph,  or  by  seismographs 
of  the  same  model,  but  not  putting  seismographs  of  different  patterns  in  motion  ; 
reported  by  experienced  observers  only. 

II.  Shock  recorded  by  several  seismographs  of  different  patterns ;  reported  by 
a  small  number  of  persons  at  rest. 

III.  Shock  reported  by  a  number  of  persons  at  rest ;  duration  or  direction  noted. 

IV.  Shock  reported  by  persons  in  motion ;  shaking  of  movable  objects,  doors 
and  windows,  cracking  of  ceilings. 

V.  Shock  felt  generally  by  every  one  ;  furniture  shaken  ;  some  bells  rung. 

VI.  General  awakening   of   sleepers;    general  ringing  of  bells;  swinging   of 
chandeliers;  stopping  of  clocks;  visible  swaying  of  trees;  some  persons  run  out 
of  buildings. 

VII.  Overturning  of  loose  objects ;  fall  of  plaster ;   striking  of  church  bells  ; 
general  fright,  without  damage  to  buildings. 

VIII.  Fall  of  chimneys ;  cracks  in  the  walls  of  buildings. 

IX.  Partial  or  total  destruction  of  some  buildings. 

X.  Great  disasters ;  overturning  of  rocks ;  fissures  in  the  surface  of  the  earth ; 
mountain  slides. 

Determination  of  the  mechanical  equivalent  of  each  degree  on  the 
Rossi-Forel  scale. 

It  is  necessary  to  determine  the  value  of  each  degree  on  the 
Rossi-Forel  scale  in  terms  of  some  natural  units.  This  it  is 
impossible  to  do  with  exactness,  owing  to  the  nature  of  the 
subject,  and  it  is  somewhat  difficult  to  get  results  sufficiently 
exact  to  be  used  in  practice. 

Referring  to  the  Rossi-Forel  scale,  we  find  that  degrees  I, 
II,  III  correspond  to  the  feelings  of  the  observer — to  his  sen- 
sations. The  rest  of  the  scale  (IY-X)  refers  chiefly  to  the 
effects  of  the  shock  in  producing  motion  upon  inanimate  mat- 
ter. The  problem  is  to  get  some  kind  of  a  common  unit  of  a 
mechanical  sort,  and  to  express  the  various  degrees  of  the  scale 
in  terms  of  this  unit.  There  is  no  question  as  to  what  unit  to 
employ.  The  researches  of  the  Japanese  seismologists  have 
abundantly  shown  that  the  destruction  of  buildings,  etc.,  is 
proportional  to  the  acceleration  produced  by  the  earthquake 
shock  itself  in  a  mass  connected  with  the  earth's  surface. 

The  earthquake  motion  is  a  wave-motion,  and  although  it  is 
not  simple  harmonic,  it  is  necessary  to  assume  it  to  be  such  to 
obtain  a  basis  for  computation.  We  assume  then  a  —  ampli- 
tude of  the  largest  wave ;  T  =  period  of  the  largest  wave  ; 

V  =  —=-  =  velocity  of  the  impulse  given  by  the  shock  ;  I  = 

—  =  47T2,     ™  =  intensity  of  the  shock,  defined  mechanically 
a 


Holden — Earthqualce- Intensify  in  San  Francisco.      429 

i 

=  destructive  effect  =  the  maximum  acceleration  due  to  the 
impulse. 

It  would  be  logical  to  express  I  in  fractions  of  the  accelera- 
tion due  to  gravity,  i.  e.,  9810mm  per  1s.  As  these  fractions  are 
usually  small,  it  is  convenient  to  give  the  values  of  I  in  terms 
of  millimeters  per  I8. 

The  observations  of  Ewing,  Milne  and  Sekiya  on  Japanese 
earthquakes  give  for  each  shock  a  and  T,  from  which  V  and  I 
can  be  computed.  Very  frequently  a  description  of  the  effects 
of  the  shock  on  buildings,  etc.,  is  given  by  them,  which  descrip- 
tion is  often  sufficiently  minute  to  justify  the  characterization 
of  the  shock  by  one  of  the  degrees  of  the  Rossi-Forel  scale. 

I  have  carefully  examined  all  the  writings  of  the  three  gen- 
tlemen named,  accessible  to  me,  and  after  rejecting  all  doubt- 
ful cases,  I  have  found  twenty-one  shocks  ranging  in  intensity 
from  I  to  IX,  in  which  the  a  and  T  were  determined  by  in- 
struments and  in  which  I  could  assign  the  Rossi-Forel  intensity 
with  confidence.  The  following  table  is  the  result : 


Equivalents  of  the  degrees  of  intensity  of  Earthquake  shocks  on 
the  Rossi-Forel  scale,  in  terms  of  the  acceleration  due  to  the 
velocity  of  the  shock  itself* 


T  _  V2 

-  a 


2-n-a 


Rossi-Forel 
Scale. 
I 
II 
III 
IV 
V 
VI 
VII 
VIII 
IX 


Intensity. 

corresponds  to  20mm  per  I" 
40 
60 
80 
110 
150 
300 
500 
1200 


Diff. 

"(20) 

(20) 

(20) 

(30) 

(40) 

(150) 

(200) 

(700) 


So  far  as  I  know,  this  is  the  best  determination  possible  from 
the  meager  data  now  available. 

The  observations  at  Berkeley  and  Mt.  Hamilton  are  espe- 
cially directed  toward  obtaining  better  values  of  these  rela- 
tions. A  few  years  of  observations  will  determine  them,  at 
least  for  the  lighter  shocks  (I-VI). 

*  It  is  interesting  to  observe  the  influence  of  long  period  in  diminishing  the 
destructive  effect  of  a  shock  of  given  amplitude.  Thus  a  shock  of  intensity  VIII 

If  T  =  0-1»,  a  =  O'lmm,  while  if 


has  1 

T  =  1s  a  =  13min 


500mm  per  !•  by  observation. 
and  so  for  other  cases. 


430      Holden — Earthquake- Intensity  in  San  Francisco. 


Absolute  intensity  of  Earthquake  action  at  San  Francisco. 

417  shocks  of  all  intensities  have  been  recorded  at  San  Fran- 
cisco in  the  years  1808-1888.  Of  these,  200  were  described  so 
definitely  that  their  intensities  could  be  assigned  on  the  Rossi- 
Forel  scale  with  tolerable  certainty.  This  work  has  been 
done  with  great  care  and  is  summarized  in  the  following  table  : 

No.  of  shocks  actually  recorded  at  San  Francisco  (1808-1888)    for  which  the  in- 
tensity is  known. 

Intensity  on  Bossi-Forel  Scale.  Number  of  Shocks. 


II    . 

4 

Ill 

55 

IV  

50 

V  

58 

VI 

12 

VII                 .     . 

4 

VIII 

7 

IX  . 

.   2 

Total, 


.200 


Beside  the  200  shocks  of  known  intensity,  there  are  217 
shocks  printed  in  my  catalogue.  No  doubt  a  great  number  of 
the  lighter  shocks  (I,  II,  III,)  are  not  recorded  at  all. 

Earthquake  action  is  so  irregular  and  lawless,  that  it  is  not 
possible  to  make  any  estimate  however  rough  of  the  number 
of  these  lighter  shocks.  Experience  has  amply  proved  that 
the  average  intensity  of  San  Francisco  shocks  is  not  above  IY 
on  the  Rossi-Forel  scale.  The  vast  majority  of  our  shocks 
are  II  and  III  and  the  average  is  certainly  below  IY.  I  shall, 
therefore,  assume  this  fact  as  a  basis  for  computation. 

The   200   shocks   of    known   intensity   are   evaluated   and 
summed  up  in  the  following  table : 


8  shocks  of  intensity 


4 
55 
50 

58 

12 

4 

7 

2 


I         correspond  to    8  x     20 

II 

4x      40 

III 

55  x      60 

IV 

50  x      80 

V 

58  x    110 

VI 

12  x    150 

VII 

4x    300 

VIII 

7x    500 

IX 

2x  1200 

Units  of  Acceleration. 

=  160 

=  160 

=  3300 

=  4000 

=  6380 

=  1800 

=  1200 

=  3500 

=  2400 


200  recorded  shocks  of  known  intensities  correspond  to  22900   units. 

The  average  recorded  shock  corresponds  to  I  =  114  units  or 
approximately  to  Y  on  the  scale.  This  simply  proves  that  all, 
or  nearly  all,  the  shocks  of  intensity  Y  and  more  severe  have 
been  recorded  and  that  the  lighter  shocks  have  been  neglected. 

As  has  been  said  417  shocks  in  all  have  been  noted  (of  which 
only  200  are  accurately  described).  I  assume  the  217  shocks  of 


Holden — Earthquake-Intensity  in  San  Francisco.      431 

unknown  intensities  to  have  had  between  48  and  49  units  of 
intensity  each,  or  10460  units  in  all.     This  amounts  to  sup- 
posing our  average  shock  to  be  of  intensity  IY. 
In  this  way  the  table  will  stand : 

Units  of  Acceleration. 

217  shocks  of  unknown  intensity  give 1 0460 

200       "       "    known  intensity  give ..- 22900 

417  shocks  recorded  (1808-1888)  give 33360 

The  average  shock  is  of  intensity  IY  corresponding  to  80 
units  or  to  y^d  part  of  the  acceleration  due  to  gravity.  The 
total  intensity  of  33360  units  has  been  experienced  in  80  years 
and  corresponds  to  3*4  the  acceleration  due  to  gravity.  That 
is  if  all  the  earthquake  force  which  has  been  expended  in  San 
Francisco  during  the  past  80  years  were  concentrated  so  as  to 
act  at  a  single  instant,  it  would  be  ^capable  of  producing  an 
acceleration  of  3 '4  times  that  of  gravity  or  about  109  feet  per 
second. 

The  total  earthquake  intensity  during  the  80  years  is  nearly 
equal  to  the  intensity  of  28  separate  shocks  as  severe  as  that  of 
1868,  but  it  has  been  doled  out  so  gently  and  gradually  that 
we  have  scarcely  known  of  it. 

On  the  average  392  units  of  intensity  have  been  developed 
during  each  one  of  the  80  years  (1808-88).  This  will  allow  for 
six  shocks  of  intensity  III  per  year  or  one  every  two  months. 
In  fact  417  shocks  have  been  recorded  in  the  960  months. 

I  believe  that  my  earthquake  catalogue  as  printed  and  the 
present  note,  contain  nearly  all  the  precise  information  which 
can  be  extracted  from  our  past  records,  at  this  time. 

The  automatic  earthquake  registers  now  in  use  at  the  Uni- 
versity of  California,  Berkeley  (under  the  care  of  Professors 
Le  Conte  and  Soule)  and  at  the  Lick  Observatory,  Mount 
Hamilton,  will  afford  valuable  data  after  a  few  years. 

I  am  greatly  in  hopes  that  the  chiefs  of  the  U.  S.  Geological 
Survey  and  of  the  TJ.  S.  Signal  Bureau  may  find  it  practicable 
to  establish  and  care  for  seismometric  stations  in  the  state. 
The  co,st  of  such  stations  is  small.  I  find  that  the  excellent 
duplex-pendulum  instrument  of  Professor  Ewing  can  be  satis- 
factorily duplicated  for  $15.  The  California  Electric  "Works, 
35  Market  street,  San  Francisco,  is  now  prepared  to  furnish 
such  instruments  at  that  price.  If  a  sufficient  number  of 
stations  can  be  established  in  California,  it  seems  to  me  that  we 
may  look  forward  to  the  collection  of  data  of  real  theoretical 
and  of  some  practical  importance  within  comparatively  few 
years. 


THE  UNIVERSITY  LIBRARY 

UNIVERSITY  OF  CALIFORNIA,    SANTA  CRUZ 

SCIENCE  LIBRARY 


This  book  is  due  on  the  last  DATE  stamped  below. 


11 1991  : 


MR  01  1995 


SANTA  CRU 
INTERLIBRARY  LOAN 
FEB23  1995REG'0 

FEB281395RBD 


Series  2477 


UC-SAJ/TA    CRUZ 


•»•  »«  •'  "Bin  gin  iiii   i|   MUI   III   III)   Ml     ||          |         | 

3  2106  00532  6670 


